1. Field of the Invention
This invention relates to a second harmonic wave-generating (SHG) element preferably usable for a device such as a blue laser source.
2. Related Art Statement
An element to generate a blue laser is suggested which is made by forming an optical waveguide having periodically polarization-inversed structure and in which an infrared semiconductor laser is introduced into the optical waveguide (U.S. Pat. No. 4,740,265, JP-A-5-289131, and JP-A-5-173213). For example, JP-A-6-51359 discloses a SHG element in which a polarization-inversed layer, an optical waveguide, a dielectric film, and a reflective grating layer are formed and a thickness of the dielectric film is defined into a given value.
Although these techniques require high-precisely controlling of domains, it is very hard to do so. An allowable temperature for the phase-matching must be controlled within a precision range of .+-.0.5.degree. C. Moreover light damage of the optical waveguide may be recognized at 3 mw and over of a light energy. Considering these phenomena, it is pointed out that these techniques have a problem as a practical device.
On the other hand, NGK Insulators, Ltd. suggested in JP-A-8-339002 a SHG element having little light damage without a quasi-phase-matching or controlling domains at a high-precision. In this literature, a single crystal substrate is made of lithium potassium niobate or Ta-substituted lithium potassium niobate by micro pull-down method, and an optical waveguide made of a material of the same kind as of that of the substrate is formed on the substrate.
The SHG element was an epoch-making element because the element could make light damage remarkably small in the optical waveguide for converting a wavelength, and thereby an outlook for providing an element for a practical use can be obtained. However, for wide use as a blue laser source, it is required that a generation efficiency of a second harmonic wave be further increased.